DMSO SOLVENT-INDUCED PHOTOCHEMISTRY IN HIGHLY PHOTOSTABLE COMPOUNDS -THE ROLE OF INTERMOLECULAR HYDROGEN-BONDING

2-(2'-Hydroxy-5'-methylphenyl)benzotriazole 1, Tinuvin P, and o-hydrox ybenzophenone, 2, are thought to achieve exceptional photostability th rough highly reversible deactivation associated with their intramolecu lar hydrogen bonds. Excimer laser excitation (10 mJ, 20 ns at 308 nm) of these molecules in argon-bubbled hexane solution at room temperatur e affords no discernible transient signals (absorption or emission) be tween 320 and 800 nm. In DMSO solution under the same conditions, howe ver, strong transient absorptions in the visible (lambda(max) 410 and 425 nm for 1 and 2, respectively) are observed. The transients respons ible for the absorptions are quenched by oxygen and acid, and the spec tra match well with the ground state difference spectra generated from the corresponding phenolate ions. When a bulky group is incorporated ortho to the hydroxyl function such as in 2-(2'-hydroxy-3'-cumyl-5'-( 1,1,3,3-tetramethylbutyl)phenyl)benzotriazole 3, Tinuvin 928, no appre ciable transient absorption is observed even in DMSO solution. These r esults are consistent with the disruption of the intramolecular hydrog en bond of 1 and 2 (but not 3) as the result of intermolecular hydroge n bond formation with DMSO. The ''dramatic'' ortho effect implies that the bulky group adjacent to the hydroxyl function sterically shields the intramolecular hydrogen bond from disruption by polar basic enviro nments.